We wish to understand how specific genes are activated in specific cells and at specific times during development. As a model system we have chosen the developing erythroblasts from chick embryos and have restricted our approach to investigating the molecular differences between genes which are actively being transcribed and genes that are dormant. Toward this end we will pursue our earlier observations that actively transcribed chromatin is much more sensitive to digestion by DNase I. Preliminary results indicate that this sensitivity is lost if chromatin is extracted with 0.35M NaC1. Reconstitution of the depleted chromatin with this fraction (which contains at least 70 proteins on one-dimensional gels) results in the reestablishment of DNase I sensitivity. We wish to isolate and characterize the component(s) responsible for inducing DNase I sensitivity of active genes. Several additional experiments are directed toward mapping the size of the DNase I sensitivie "patch" of chromatin surrounding the globin gene. Because the mechanism of activation of the globin gene during development may be analogous to the mechanism by which already active genes become packaged into active nucleosomes after chromosome duplication, we will continue or analysis of how chromosomal proteins assemble and segregate during gene replication and, specificially, to continue our earlier studies demonstrating that chromosomal proteins segregate conservatively during chromosome replication.